The aim of the article is to present the Mathematics Attitude Scale for Adults (MASA) - a new, three-dimensional questionnaire of attitude towards mathematics for adults which contains cognitive, affective, and behavioral components. The exploratory and confirmatory factor analyses established a three-factor structure of the MASA and measurement invariance across gender and groups of students and non-students. The convergent and divergent validity was confirmed, which shows that women have a more positive math attitude than men, and that STEM students have a more positive attitude than humanistic/social science students and non-students. The results also showed that a more positive math attitude is related to lower math anxiety and higher math self-concept, math self-efficacy, math grades, and math state final exam. Moreover, a higher MASA score is related to lower results in Polish language self-concept, Polish language self-efficacy, Polish language grades, and Polish language state final exam. Satisfactory internal consistency and test-retest reliability was also demonstrated. The MASA has very good psychometric properties. Therefore, it can be recommended to investigate math attitude in various groups of adults.